Communication systems commonly have antennas mounted on top of towers. The antennas send radio signals to, and receive radio signals from, remote communication equipment such as radios, mobile telephones, and the like. In a cellular telephone application, for example, radio waves from the cellular telephone are received by an antenna mounted on top of a tower, or similar structure, which converts the waves to an electrical signal. A transmission line or cable then carries the electrical signal to a base station for processing. The base station is typically positioned at the base of the tower and includes radio equipment such as a receiver for receiving a radio signal from the cellular telephone and a transmitter for transmitting a radio signal to the telephone, thereby establishing two-way communication. The radio signals are processed and communicated to either another cellular telephone or a traditional land-line telephone so that the people can speak to one another.
One problem with these systems is that the radio wave from the cellular telephone that is received at the base-station antenna, and hence the resulting electrical signal, is relatively weak. The electrical signal received needs to be amplified so that it can be carried along the transmission line from the top of the tower to base-station equipment for processing. Accordingly, some installations place an amplifier at the top of the tower to amplify the electrical signal before it is communicated down the transmission line. In one possible application this amplifier is a low-noise amplifier to reduce the ratio of noise to signal amplitude, which increases sensitivity of a system.
These amplifiers are prone to failure for several reasons. For example, they are exposed to extreme weather conditions including freezing cold and high heat. Furthermore, amplifiers that handle high power or have a high gain produce heat that can degrade the semiconductor material from which they are made. This degradation in the material also causes failure. The difficulty is that a failed amplifier can cause the transmission tower to become inoperative and result in an entire geographic area losing cellular telephone service. Given the reliance on cell phones for business and emergency workers, this loss could be catastrophic.
Furthermore, climbing a tower can be dangerous, especially during winter when it is cold and icy. This danger is especially true for some towers that can exceed a height of 400 feet. As a result, technicians need to avoid climbing towers except to perform scheduled maintenance, which is preferably scheduled during good weather and temperate times of the year.
One approach to minimize the impact of an amplifier that fails is to provide redundant amplifiers. In this approach, switches are positioned upstream and downstream from the amplifiers. The switches then control to which amplifier the electrical signal is input. A shortcoming with this approach is that the electrical signal that is carried to the base station for processing must flow through the switches, which introduce noise into the signal and cause insertion loss that decreases the strength of the signal and counters the effect of the amplifier.
Additionally, one possible configuration has balanced amplifiers in which the signal is split so that a first portion of the electrical signal passes down one signal branch through a first amplifier and a second portion of the electrical signal passes down a second signal branch and through a second amplifier. The signals are then recombined and carried by the transmission line to the base station. In this manner, the pair of amplifiers cooperate to condition the signal received from the cellular phone. If a redundant amplifier is used, the switches switch between pairs of amplifiers, which also has shortcomings. For example, if one amplifier in the main pair fails, the switches will direct the radio signal to the redundant pair of amplifiers. The good amplifier in the first pair then sits idle and cannot be used until its mating amplifier is repaired. This configuration limits the number of potential combinations between amplifiers and thus limits how much the failure rate can be reduced through the use of redundant amplifiers.
Accordingly, there is a need in the art for an amplifier that has a reduced risk of failure. There is a related need for an amplifier that has increased redundancy. Accordingly, there is a related need for a balanced amplifier configuration that maximizes the number of possible combinations between amplifiers. There is yet another need for a redundant amplifier in which the electrical signal being processed does not flow through a switch. There is a related need for a redundant amplifier that has improved noise and sensitivity characteristics.